TB is a chronic, necrotizing infection caused by M. tuberculosis. The clinical manifestations of disease are the result of a balance between the host response and bacterial virulence. Cellular immunity is responsible for effective control of infection, but cytokines released during the process of cellular immunity may also cause harm to the host. Humoral immunity plays little part in protection against TB. Individuals with defective cellular immunity are much more susceptible to disease from M. tuberculosis and are more likely to have a disseminated form of TB.